The purpose of this research is to describe the role of certain reflex systems in the control of unrestrained, functional human behaviors. The specific behavior to be examined is stepping-down - an act necessary for full mobility. Healthy adult volunteers will participate in the experiment which will be organized around the biomechanical and electrophysiological parameters of interest. These are a) force of the impact on the lower level, b) downward velocity of the body, c) angle at which the ankle first contacts the lower level, d) acceleration of the head during the steps, e) EMG activity of the lower limb musculature throughout the step-down procedures. The experimental conditions examined are: (1) trials at several step heights under normal sensory conditios, (2) trials without visual cues (subject blindfolded and not informed of the step height), (3) trials in which a sensory conflict is caused (between visual and vestibular senses) by the visual surrondings moving with or opposed to the stepping subject. Knowledge of the stresses incurred during step-down will offer an important basis for re-evaluation of the mobility needs of the partially disabled and geriatric populations. More generally, the data will indicate the importance of considering human reflex systems not with the person in a passive state, but rather during performance of active behavior because reflex responses are dependent upon the functional state of the organism.